1. Field of the Invention
The present invention relates to an optical waveguide device in which a glass optical waveguide is integrally formed on a dielectric or semiconductor substrate.
2. Description of the Related Art
As optical waveguide devices, there are conventionally known an optical waveguide serving as a passage for light transmission, an optical modulator for modulating the intensity and phase of light propagating in the optical waveguide (hereinafter, such light is referred to as waveguide light), an optical switch for switching on/off the waveguide light, a waveguide type optical amplifier for amplifying light, and an optical phase matching device.
These conventional optical waveguide devices are described, for example, in R. Alferness, "Waveguide Electrooptic Modulators", IEEE Transactions on Microwave and Techniques, Vol. MTT-30, No. 8 pp. 1121-1137 (1982).
These optical waveguide devices are fabricated by forming an optical waveguide on a dielectric substrate, a glass substrate, or a semiconductor substrate. I. Kaminow, "Optical Waveguide Modulators", IEEE Transactions on Microwave and Techniques, Vol. MTT-23, No. 1, pp. 57-70 (1975) describes various conventional methods for fabricating these optical waveguide devices.
For example, the above mentioned document describes fabricating methods in which an optical waveguide is formed on a dielectric substrate, especially a dielectric substrate having an electrooptic effect. Examples of these methods include: a method in which Ti is diffused in a substrate made of lithium niobate or the like and the Ti-diffused portion is used as the optical waveguide; a method in which a crystal of lithium niobate is grown on a substrate made of lithium tantalate by epitaxy and the crystal is used as the optical waveguide; and a method in which a thin film of lithium niobate is formed on a substrate made of lithium niobate or lithium tantalate by sputtering and the thin film is used as the optical waveguide.
The optical waveguide device fabricated as described above using a dielectric substrate is advantageous in that it responds at high speed and that the electrical control of the waveguide light is easy. It is therefore used as a high-speed optical modulator. However, since the shape, size, and diffractive index of the optical waveguide at its coupling portion greatly differs from those of an optical fiber to be coupled, the loss of light at the coupling of the optical waveguide with the optical fiber (hereinafter, referred to as the "coupling loss") is great.
Other methods for fabricating the optical waveguide device in which an optical waveguide made of glass or quartz is formed on a semiconductor substrate or other various types of substrates are also known. Examples of these methods include: a method in which a silicon oxide film is formed on a semiconductor substrate made of Si by thermal oxidation and then an optical waveguide is formed on the silicon oxide film; and a method in which glass or quartz material is deposited on an Si substrate by various thin film technologies such as sputtering, vacuum evaporation, chemical vapor deposition, flame hydrolysis deposition, and sol-gel transformation, and then an optical waveguide is formed on the deposited glass or quartz layer.
For example, Japanese Laid-Open Patent Publication No. 1-189614 discloses a structure fabricated by a flame hydrolysis deposition method which includes the steps of: forming a clad layer made of glass or the like having a refractive index lower than an optical waveguide to be formed in a later step, forming a layer (core portion) made of glass or quartz material having a refractive index higher than the clad layer on the clad layer, and, if required, forming another clad layer having a refractive index lower than the core portion on the core portion, thereby controlling the confinement of light into the core portion. This structure allows light to be substantially completely confined in the core portion, and thus can be used as an optical waveguide device.
The above optical waveguide device fabricated by the thin film technology has problems as follows:
(1) Using glass or quartz material for the optical waveguide is preferable because, in general, the loss of light during the propagation decreases as the purity and the solidity of the material are higher. However, in the thin film technology described above, it is generally difficult to control the quality and refractive index of a resultant thin film. Accordingly, it is difficult to obtain a core portion with a low loss of light during the propagation.
(2) When the optical waveguide is coupled with an optical fiber, the coupling loss is smaller as the shape and refractive index of the optical waveguide are closer to those of the optical fiber. However, the core diameter of a single-mode optical fiber is in the order of 10 .mu.m, and any thin film technology finds it difficult to form a high-quality, uniform thin film having a thickness as large as 10 .mu.m. Moreover, even when a material of the same composition as that of the optical fiber is used for the thin film, the refractive index of such a thin film formed by the thin film technology is not necessarily identical to that of the core portion of the optical fiber formed by the glass formation technology including melting and solidifying. Accordingly, it is difficult to match the refractive indices therebetween. Furthermore, forming a thicker film by the thin film technology results in low productivity. In addition, since the variation of materials available for the thin film is greatly limited, the function and design for the optical waveguide is limited.
The fabricating method using the silicon oxide film formed on the Si substrate by thermal oxidation also has a problem. Since the oxide film is formed at a rate of 0.1 .mu.m/hour at 1000.degree. C., it takes a long time to form a thick film, which also lowers the productivity.
U.S. Pat. No. 5,193,137 discloses a structure in which a glass optical waveguide is formed on a glass substrate. More specifically, a quartz glass core portion is formed on a quartz glass substrate with a glass layer having a low refractive index interposed therebetween.
The above glass optical waveguide device is advantageous in that, since the shape, size, and refractive index thereof can be easily matched with those of an optical fiber, the coupling loss of the optical waveguide with a typical quartz optical fiber can be reduced. However, since glass itself does not have the electrooptic effect, the electrical control of the waveguide light is impossible by this glass optical waveguide device. To overcome this problem, there is proposed a method of controlling the waveguide light by using a change in the refractive index caused by heating or the like. However, the response speed of the resultant device is critically low compared with that of the device using the electrooptic effect.
There is also known a glass optical waveguide device in which an optical waveguide is formed by changing the refractive index by diffusing metal. In such a device, however, the shape at the coupling portion of the optical waveguide is less symmetrical, and therefore is not so effective in reducing the coupling loss with the optical fiber. Moreover, in the optical waveguide formed by metal diffusion, the loss of light during the propagation increases due to the diffused metal.